1. Field of the Invention
This invention relates to a reflow soldering method, and particularly to a reflow soldering method employing a solder paste which does not require removal of flux residue after soldering.
2. Description of the Related Art
There is a constant demand by consumers for decreases in the size and weight of electronic equipment. Coping with this demand requires decreases in the size of electronic components contained in such equipment and increases in the packaging density of such components. For this reason, there has been a renewed interest in flip chip technology for mounting electronic components.
In flip chip technology, first developed in the 1960's, a semiconductor chip is placed face down on a substrate (such as a printed circuit board), and terminations on the lower surface of the chip are electrically connected to the upper surface of the substrate. A commonly used method for electrically connecting a flip chip to a substrate is to form solder bumps on the chip and reflow the solder to thereby join the chip to the substrate.
In the past, solder bumps have been prepared by electroplating. However, as the size of solder bumps decreases, especially with lead-free solders, it becomes difficult to form solder bumps by electroplating on an industrial scale due to the high cost of electroplating and the difficulty of forming a large number of solder bumps having a uniform alloy composition by electroplating.
A conceivable alternative to electroplating is to apply a solder paste to a member by printing and then reflow the solder paste to form it into solder bumps. Printing is economical and enables the formation of bumps of good uniformity. A typical solder paste for use in printing comprises a solder powder and a flux. The flux imparts printability to the paste, and it contains one or more active components (activators) for reducing oxides on the surface of solder or the member to be soldered and for increasing the wettability and spreadability of the solder.
With many fluxes, a flux residue remains on the member being soldered at the completion of soldering. The active components in the flux residue are frequently corrosive, so it is necessary to clean off the flux residue to prevent damage to the member being soldered. In the past, flux residue was often cleaned off using a cleaning fluid comprising a chlorofluorocarbon-containing solvent, but the use of such solvents has now been significantly restricted due to their adverse effects on the ozone layer. Therefore, the removal of flux residue has become more challenging than in the past. Furthermore, regardless of what type of cleaning fluid is employed, it can be difficult to completely remove flux residue from a substrate when the spacing between soldered components on the substrate and the spacing between the components and the substrate are extremely small, as is frequently the case with flip chips.
Accordingly, in order to form solder bumps for use with flip chips economically and on an industrial scale, it is important to be able to apply a solder paste to a member by printing without the solder paste leaving a flux residue after reflow soldering. For this reason, research is now being carried out with respect to fluxless soldering, which is soldering not employing a flux.
One method of fluxless soldering which has been proposed comprises applying a fluxless solder to a substrate by plating or vapor deposition and then reflowing the solder to form the solder into bumps while exposing the solder to a plasma, such as a hydrogen plasma Such a method is described in Japanese Published Unexamined Patent Application Hei 11-163036, for example. Free radicals in the plasma exert a reducing action on oxides in the solder and can therefore serve the purpose of the active components in a conventional flux. Since the solder does not contain a flux, there is no formation of flux residue, so there is no need to perform cleaning after soldering to remove flux residues. However, the need to apply the solder by plating or vapor deposition makes the method uneconomical and makes it difficult to uniformly apply the solder, so it is not truly practical as an industrial method. Thus far, there have been no proposals of methods employing the use of a plasma while permitting solder to be applied to a surface by printing.